The present invention relates generally to radiological imaging, and more particularly, to image compression apparatus and methods for use in compressing and reconstructing radiological images.
The medical imaging industry is moving away from traditional film-based systems and is moving toward the use of digital images presented on computer work-stations. The images that are presented to radiologists must preserve the information contained in the original film images in order for soft copy images to be accepted by the medical community. The typical radiological image is different from other images in several ways.
Radiological images used in medicine present peculiar problems for compression compared to conventional image compression problems. The images locally have very low contrast. For example, a transition over an edge of a bone does not change the image intensity by a large amount. While the local variations are small, the dynamic range over the whole image may be quite large. Much of the information is contained in the local variation of image intensity. The overall image provides a context within which the information is embedded, but the pathology is visible as local variations. The images are subject to large changes in the individual pixel intensity as the radiologist views the image. A radiologist may change the contrast and the center of the contrast range in his efforts to see details in the image. In the extreme, the radiologist may invert the image, changing white to black. The images are very large, typically more than 1 k by 1.2 k pixels and up to 2 k by 2.5 k pixels. The image dynamic range typically requires 12 bits. The image is viewed at a number of magnifications from 2:1 minified, to 8:1 zoomed.
The well-known CCITT JPEG standard uses an 8.times.8 discrete cosine transform in the image compression scheme. The number of bits in the image is 8 or 12 bits. The blocking effect of the discrete cosine transform is unacceptable as an artifact over the entire image. The dynamic range of 12 bits cannot be squeezed into 8 bits. If a typical JPEG compressed image is examined in detail, the blocking of the 8.times.8 discrete cosine transform is visible over the whole of the image. The details of the image are visible through the blocking, but the distraction of the blocking over the whole of the image is unacceptable. The radiologist often looks at a zoomed image with contrast arranged so that the blocks interfere with his viewing of the image.
If radiological images were viewed in the same way that a TV image is viewed, the dynamic range could be compressed to 8 bits by a simple nonlinear mapping of image intensities. A typical mapping scheme is logarithmic mapping. With this scheme, displayed image intensities are proportional to the logarithm of the original image intensities. Since the image contrast and intensity center can be varied, including inverting the pixel values, such nonlinear mapping is not appropriate. No companding of the image intensity values is tolerable, since the compression from 12 bits to 8 bits introduces large artifacts at some level of contrast or center for the displayed intensity values.
Other techniques that are used for image compression are typically full image compression schemes. Either a Fourier transform or a cosine transform is used over the full image. Since the transform is a full image transform, there is no blocking of the image. The difficulty with the full image transform is that the amount of computation per pixel is large and the images, in many cases, are constrained to be square images. If the images are not square, they are padded to make them square. In many implementations, the number of pixels in the image that is compressed is constrained to be a power of two. Again the images must be padded with zeros to make them a power of two.
In view of the above, it is an objective of the present invention to provide for apparatus and methods that provide for image compression of radiological images that suppresses artifacts present in the image due to blocking.